Recently, in order to protect the environment, a decrease in the global emissions of carbon dioxide has been sought. In the automobile industry, there is an active effort to decrease carbon dioxide emissions from internal combustion engines by introduction of electric vehicles (EV) and hybrid electric vehicles (HEV) powered by electric motors. This has led to recent progress in the development of lightweight and lower cost secondary storage batteries for powering electric motors.
Although some secondary storage batteries (e.g. lithium ion batteries) can achieve high energy density and high output power density, it is generally necessary to serially connect multiple batteries in order to obtain a sufficiently large output power to power an electric motor vehicle. However, when batteries are serially connected through an external electrical connection (e.g. external wiring), the output power is decreased due to the electrical resistance of the external electrical connection. Furthermore, batteries having an external electrical connection are spatially disadvantageous. In other words, due to the length of the external electrical connection, the output power density and energy density of the batteries are decreased, and the overall operating efficiency of the electric vehicle is reduced due to the weight of the external electrical connection.
To solve this problem, bipolar cells have been developed in which a positive electrode (i.e. cathode type) active material and a negative electrode (i.e. anode type) active material are placed on opposites sides of a collector (see, for example, Japanese published unexamined patent application No. 2004-95400). Multiple bipolar cells may be electrically connected internally to form a secondary storage battery module (i.e. a bipolar electrode battery). Such bipolar electrode batteries are known to exhibit good electrical performance characteristics and desirable physical properties, such as low weight to volume ratio, thin cross-section, and good heat dissipation rate during operation and charging.
Because of the rapid adoption of the bipolar electrode battery by the automotive industry, it has been possible to improve the output power density and energy density of batteries for motor vehicles. However, additional improvement of bipolar cell battery performance is continually sought.